1. Field of the Invention
The present invention relates to a method of manufacturing steel plates which are useful in fabricating pressure vessels, and more particularly it relates to improved chromium-molybdenum type steels which can be formed into thick plates that are weldable together to make chemical pressure vessels.
2. The Prior Art
Chemical pressure vessels, such as pressure vessels used as oil-refining reactors, must be fabricated from materials which excel in high temperature strength, creep strength and hydrogen-attach resistivity. The most widely used materials have been 11/4Cr-0.5Mo type steels, which are standarized steels as discussed, for example, in ASTM A387 G11.
In recent years, however, pressure vessels have been growing larger and larger in size, thus requiring the use of steel plates having increasing thicknesses (e.g., of 100 mm or more). Conventional 11/4Cr-0.5Mo type steels, which are generally produced using normalizing and tempering treatments, have not produced completely satisfactory steel plates having such great thicknesses. This is due to the fact that as the plate thicknesses increase, the steel plate cooling speed is reduced and the ferrite content will increase during the normalizing treatment. The temperature of the stress relief annealing after welding or post weld heat treatment (hereinafter abbreviated as PWHT) thus must be elevated and the treatment duration lengthened. As a result, the strength of the produced steel will be drastically lowered. Consequently, the produced steel plates will often times not have the required strengths for use in making pressure vessels. At the same time, however, increasing the content of alloying elements such as chromium and molybdenum will not solve the problem because the weldability of the steel plates will be reduced.
In addition, it has recently been discovered that the occurrence of creep damage in pressure vessels in welded heat affected zone (hereinafter abbreviated as HAZ), accompanied by stress concentration, has become a serious problem.
Investigations on the creep damage have revealed that at normal operation temperatures of pressure vessels (400.degree.-550.degree. C.), the hardened zones suffer from much degradation of grain-boundary strength, and that owing to the concentration of strain in the grain boundaries, cracks develop at the boundaries which grow and mature into ruptures. The most effective way to prevent creep ruptures is to enhance the strength of the base metal and to soften the HAZ by applying a high-temperature PWHT (650.degree.-720.degree. C.). With respect to enhancing the strength of the base metal, this can be achieved by lowering the phosphorus content. However, drastically decreasing the phosphorus content (e.g., to less than 0.010%) entails huge cost increases because a special treatment step in the steelmaking operation becomes necessary.
One proposal for solving the noted problems is disclosed in JA-OS No. 41962/1980 (laid open for public inspection on Mar. 25, 1980). In this proposal, the employed Cr-Mo steel plates have aluminum and boron or boron and titanium added thereto. However, this proposal is basically directed to 21/4Cr-1Mo type steel plates which suffer from severe embrittlement from tempering treatment, and it is also directed to preventing this embrittlement by lowering the silicon and manganese contents. When this prior art proposal for improving steel properties is applied to 11/4Cr-0.5Mo steel plates which have thicknesses exceeding, for example, 50 mm, the steel plates will have insufficient strengths and toughnesses because of the low manganese contents. Indeed, because the silicon contents are kept low, the chromium and molybdenum contents must be increased to achieve the necessary strengths. This detrimentally increases the cost of production, and the produced steel plates still lack the extra strength needed for withstanding the high temperature PWHT and do not show improved creep rupture resistivity.
It is thus an object of the present invention to provide an improved method of producing Cr-Mo type steel plates which can be formed into extremely thick plates for use in fabricating pressure vessels, which plates will have increased strengths (even after high temperature PWHT) and enhanced creep rupture resistivity, yet will not have reduced weldability or increased cost.